In the production of plasmas, which are used for coating materials and other purposes, microwaves are increasingly being used, which in conjunction with magnetic fields produce an electron cyclotron resonance and hence a heightened ionization of atoms or molecules. Of special interest are those plasma sources which are configured very uniformly and over a great breadth so that many small or a few large areas can be coated simultaneously. In particular, plasma-enhanced chemical deposition from the gas phase, called PCVD(=plasma enhanced chemical vapor deposition), which is used as an alternative or supplement to the known sputtering or vapor depositing technique in large-area coating, requires a very uniform low-pressure plasma, at least in the direction of the greatest reach of the coating apparatus.
A microwave broad-beam ion source is known, in which the microwaves are fed through an E.sub.01 round waveguide to an E.sub.010 cavity resonator which is surrounded by a magnet coil and filled with the gas to be ionized (DD Pat. 248 904). The bottom of the cavity resonator forms an emission electrode which, together with an extraction electrode, forms an extraction system to which a vacuum chamber is connected. It is a disadvantage of this ion source that the microwave energy is not radiated uniformly by the round hollow waveguide into the cavity resonator. Since the round hollow waveguide is smaller than the cavity resonator, the microwaves radiate only in a relatively small range.
To regularize large-area microwave injections, cavity resonator antennas are known, which have round or rectangular openings in the resonator (Jacobsen, S. et. al.: An Antenna Illuminated by a Cavity Resonator, in: Proc. of the IEEE, November 1963, pp. 1431 to 1435, DE-A-35 30 647), or slits (GB-A-654,224, U.S. Pat. No. 2,996,715, DE-C-29 00617, U.S. Pat. No. 4,512,868, Swiss Patents 370,177, 368,248, 363,742, Patent Abstracts of Japan, E-570, Jan. 7, 1988, Vol. 12/No. 3, publication No. 62-165,403) or quartz windows (EP-A-0 183 561, EP-A-0 286 132). A transfer of electromagnetic energy from the cavity resonator into a spatially separate plasma chamber, however, is not possible with these known apparatus.
A plasma etching apparatus is known, which has a microwave generator which is coupled to a rectangular waveguide (DE-C-27 16 592, FIG. 1). In a position which is about 1/4th of a waveguide wavelength from the end of this rectangular waveguide, the end of the inner conductor of a coaxial waveguide extends into the hollow waveguide. Through the antenna formed in this manner the microwave of the waveguide is propagated also in the coaxial waveguide. Furthermore, an electrical field of microwaves propagates itself through an insulator as well as a coupling device between the microwaves and the plasma in a discharge current. The hollow waveguide which is in communication with the microwave generator is not a cavity resonator. Furthermore, only one inner conductor is provided and is looped directly through the dividing wall between the hollow waveguide and the discharge current.
Lastly, an apparatus is also known for producing microwave plasmas of great breadth and regularity, which has a microwave generator, a cavity resonator, a coupling to couple the microwave generator to the cavity resonator, a chamber defined by walls, a dividing wall between the cavity resonator and the chamber, and a coupling system consisting of a plurality of coupling elements in the form of solid bodies (DD-A-263,648). The coupling elements in this case are metal coupling hooks which extend out of the waveguide carrying the microwave into another waveguide adjoined by a plasma chamber. An important component of this known microwave plasma supply is the line terminated by a matched absorber, which connects or couples the coupling elements arranged in a row on the vacuum side and plasma side, respectively. This coupling constitutes a disadvantage because the individual microwave antennas should be as uncoupled from one another as much as possible, not coupled together. The coupling of the antennas on the plasma side defeats the decoupling that is achieved by supplying the antennas from a resonator of high quality. Around the individual antennas areas of intense plasma form, in which the electron density reaches its critical level of 0.75.times.10.sup.-12 cm.sup.-3 which is critical to its excitation frequency. This means that a great part of the power fed into the antenna on the vacuum side by the plasma is completely reflected.